2
64
R.M. El-Shazly / Spectrochimica Acta Part A 74 (2009) 259–264
Table 5
Kinetic and thermodynamic parameters of the thermal decomposition of Fe(III) and Cr(III) complexes.
◦
Compound
Step ( C)
Coats–Redfern eqn.
Horowtiz–Metzger eqn.
ꢅS*
ꢅH*
ꢅG*
R
n
E
r
n
E
[Fe(L)3]
0.9972
0.00
0.33
0.66
1.00
2.00
0.00
0.33
0.66
1.00
2.00
79.9
89.9
100.4
11.25
154.30
72.50
83.40
95.40
109.30
158.10
0.9966
0.9983
0.9994
0.9999
0.9988
0.8959
0.9906
0.9941
0.9967
0.9992
0.00
0.33
0.66
1.00
2.00
0.00
0.33
0.66
1.00
2.00
89.10
98.90
109.80
122.3
−180.00
75.80
85.50
96.30
108.40
150.20
66.80
77.70
89.70
103.60
152.40
164.50
163.40
16.40
164.50
158.00
221.30
234.80
234.20
250.10
228.10
0
0
1
0
0
0
0
0
0
.9988
.9997
.000
.9978
.9857
.9912
.9952
.9977
.9988
−158.00
−134.10
−113.90
−15.90
1
4
st
93
164.9
82.60
93.60
105.70
119.40
168.00
−224.60
−228.40
−210.00
−212.90
−110.10
3
6
rd
88
0
0
.9684
.9763
0.00
0.33
0.66
1.00
2.00
89.20
104.00
120.30
138.80
205.70
0.9712
0.9777
0.9832
0.9877
0.9960
0.00
0.33
0.66
1.00
2.00
97.10
111.30
127.30
145.60
211.40
−195.90
−167.80
−139.80
−130.90
21.6
84.30
99.10
115.40
133.90
200.80
200.30
198.40
198.20
211.4
2
5
nd
92
[Cr(L)3(H2O)3]
0.9880
0
0
.9881
.9968
188.00
E, ꢅH* and ꢅG* are in kJ mol 1, ꢅS* in J mol−1; the underlined values of r represent the best fit values.
−
stability of such chelates due to their covalent bond character [36].
The positive sign of ꢅG for the different decomposition steps indi-
cates that the free energy of the final residue is higher than that of
the initial compound, and hence all decomposition steps are non-
spontaneous processes. The negative ꢅS values are due to the high
value of mid point temperature of the decomposition step (T) and
the low value of pre-exponential factor (A). Applying Eq. (10) for T
and A, negative values were obtained indicating more ordered for
the activated complex than the reactants or the reaction is slow
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